The advent of down-sized automobiles has also encouraged the production of smaller, more compact engines and transmissions for such vehicles. In this regard efforts have been made to decrease the size of transmissions even further. Because the torque transferring devices--i.e., the brakes and clutches--within the mechanism of a vehicular transmission are typically operated by hydraulic pressure, one effort to reduce the size of the transmission has resulted in the utilization of bores through the various shafts within the transmission, the bores serving as passageways in the hydraulic system by which pressurized fluid is directed to, and exhausted from, the torque transfer devices. Concomitant efforts have also been directed to the provision of passageways within the casing that houses the transmission mechanisms--such passageways in the casing also being incorporated to convey pressurized fluid utilized by the hydraulic system of the transmission. In the continuing effort to decrease the size of the transmission, the cover for at least one end of the casing in which the transmission is housed has also been designed in various configurations to provide passageways therein which are included within the hydraulic system serving the transmission.
One particular prior art technique involves casting a transmission cover so that the end wall thereof is relatively thick. The relatively thick wall is then machined to incise a plurality of recessed pathways across the outer surface of the end wall. The end wall is then appropriately drilled to effect communication between the pathways and the remainder of the hydraulic system. A closure plate is secured over the recessed pathways to close each pathway from adjacent pathways as well as from atmosphere. This prior art technique has been widely adopted, but it does have some drawbacks which include: (a) an increased cost for the material used to form the thicker-than-normal end wall in the cover; (b) an increased cost for machining the recessed pathways and for precisely, and accurately, drilling connecting bores from the pathways through the end wall to communicate with appropriate portions of the hydraulic system interiorly of, or within, the casing; (c) the need to provide a closure plate to cover the recessed pathways; and, (d) the need to employ seals between adjacent pathways, as well as between the pathways and atmosphere, to reduce not only the undesirable cross-communication between adjacent pathways but also the undesirable loss of hydraulic fluid to atmosphere.
Closed passageways have also been provided in covers for transmission casings by the use of an expendable core when casting the cover. This approach, however, has also proven to be disadvantageous, particularly in view of the increased unit cost resulting from the additional steps required to make, and remove, the expendable core.